A refractor is a known ophthalmic instrument having batteries of trail lenses used to determine and remedy the refractive errors of a patient's eye. In modern refractors there are similar left and right batteries which include lens disks or cells containing lenses of spherical and cylindrical power, means for rotating the lens cells to place a lens or a combination of lenses before the eye under examination and a means for setting the axis of the cylinder lens. A modern refractor also includes a synchronized Jackson cross cylinder for use as a check test for the neutralization of astigmatism (cylinder power and axis), as well as auxiliary lenses.
Techniques for refracting utilizing the Jackson crossed cylinder and the various spheres and cylinders of the refractor are well know. As refraction is presently practiced, the patient's refractive error is expressed in a sphere and a cylinder at a certain axis, which in reality represents the spherical equivalent plus the final crossed cylinder at a certain axis that is required to neutralize the patient's refractive error.
Present refractors are designated as having either positive or negative cylinders. Techniques of refracting have been designed to utilize the negative cylinders (negative theory of refracting). The negative theorem can be converted for the use of positive cylinders (positive theory of refracting) but is awkward. Similar techniques for retinoscopy have been developed mainly for positive cylinders. Both the negative and positive cylinders can be utilized in a manifest refraction when a meridional straddle is maintained. This final manifest refraction is the most accurate, as well as the most time consuming part of the refraction technique. It is the manifest refraction from which the final refraction is determined in cooperative patients, which probably comprise 95% of the average ophthalmologist's and optometrist's refraction cases.
A well known problem for the refractionist in performing the manifest refraction is maintaining a meridional balance throughout the manifest refraction. In the technique commonly performed with the conventional refractor, the refractionist is required to move one spherical lens and two cylindrical lenses in order to show the patient two images which are different by a minimum cylindrical correction.
However, a major optical error is introduced by conventional refractors during refraction of patients having an astigmatic error in accordance with this technique. A 0.125D spherical equivalent jump of the images presented to the patient occurs when such refractors are used in the final manifest refraction by presenting successive cylinder lenses. This spherical equivalent jump occurs because cylinder lenses have a spherical component (equal to one-half of the cylindrical component of opposite sign), and conventional trial lenses are graded in 0.25 diopter increments. Thus, successive cylinder lenses change the resulting spherical component by 0.125 diopter, which means the exact meridional balance can be retained only with every other cylinder increment. The problem for the patient is that the two crossed cylinder images presented to him are different by a spherical equivalent of 0.125D, and produce an inequality in image shape, i.e. the circle of least confusion becomes oval. That is, if one crossed cylinder image is a circle, the other image is an oval. The images are therefore dissimilar. (This visual comparison is obvious when a video camera, which is made astigmatic, is refracted. One image is in focus and the other is out of focus.) With the negative cylinder technique of refracting, accommodation is introduced every other time the cylinder power is changed, and with the positive cylinder technique of manifest refracting, a fog is introduced every other time the cylinder power is changed.
The refractionist can make the image comparisons for the patient equal or constant, that is comparing circles to circles or ovals to ovals by introducing a 0125D sphere auxiliary lens. However this requires the refractionist to change five lenses (two auxiliary lenses, one spherical lens and two cylindrical lenses) in order to show the patient just two similar images of no spherical equivalent difference and of a circle of least confusion that is decreasing or increasing in size. Thus, a technique utilizing an auxiliary lens in this manner is impractical and confusing in practice.
The U.S. patent to Klein et al, U.S. Pat. No. 4,385,813, teaches a computerized refractor using sphere and cylinder lenses and intended to solve the refractor manipulation problems presented by conventional refractors, but this approach is expensive and does not prevent the 0.125D spherical jump described above, although it could perhaps be adapted to accomplish that with different programming or different lenses.
Another problem that the refractionist has is the inability to maintain the same meridional balance while using the Jackson cross cylinder in order to check the cylinder power. If a fog is produced, as with the positive cylinder phoropters, the refractionist may be inclined to prescribe too much against-the-rule astigmatism. Theoretically, it is also possible to prescribe too much with-the-rule astigmatism when negative cylinders are used with a Jackson crossed cylinder in an eye which has had a cycloplegic.
Another problem the refractionist has is the inability of many patients, especially older ones, to respond to the use of astigmatic dials. The reason for this difficulty is obvious when one realizes that the theory of the astigmatic dial is based on the conoid of Strum, which exists only in a thin lens system, whereas the eye is a complicated thick lens system.
An additional problem that the refractionist has is teaching a technician or student to refract, which takes many years of experience. Computerized objective/subjective refractors have reduced this obstacle; however their cost is high, their accuracy debatable, and it is questionable whether such refractors increase refraction efficiency.
The mechanics of moving the lens wheels of the phoropter is most confusing and difficult to teach technicians and opthalmologists. It is very important to the refractionist to be able to maintain the same system of lens changes when he increases and/or decreases the crossed cylinder powers while maintaining the spherical equivalent.
An article by W. S. Dennett, "The Stokes' Lens for Measuring Astigmatism" published prior to 1900, describes utilization of variable crossed cylinder lenses or "Stokes Lenses" in connection with a mounted trail frame to refract patients. The Dennett structure does not, however, present numerous advantages achieved by the present invention, and the Stokes lens has been little used in refraction despite its long availability.